As a first step, you may need to re-seat the connector cable of the SIM tray to the board.
If your are not sure which one it is, take the phone apart, examine every cable/connector on the board and/or just re-seat them all.
If you know which one it is, try also cleaning that connector with a cotton swab and 90%+ isopropyl alcohol (rubbing alcohol).
If the phone works with a different SIM (or as a precautionary step), clean the contact points of your original SIM card with an eraser or isopropyl alcohol. May be it collected something from your fingers during disassembly.
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